kj_mustang
Well-known member
My spray foamed building is cooling about 1200 sq feet with 18,000 btu central ac unit. Either the insulation/air sealing is inadequate or that unit isn't working right.
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Contractor undersized mini split in garage, trying to figure out solutions
- Thread starter dockdoc
- Start date
Brent T
Well-known member
I assume the garage doors are insulated? There's a lot of leakage and radiant heat through and around most garage doors. When I ran the calculator to cool my 800 SF garage with 11' ceilings, I ended up with a 24k unit. The calculation came in lower, but when I started answering questions about climate, garage doors, windows, etc., it said I needed a bit over 20k BTU. On an extremely hot day in Phoenix, forget it, I can't get it below 90 degrees out there. On a "regular day" when it's below 105 outside I can cool it just fine. I did the drill and fill for the walls and had 18" sprayed in the attic plus DIY insulated the garage doors.
It's a single garage door, and yes insulated... The downstairs part actually has no problem staying cool - but granted, I keep the temp set higher (78). There's no windows down there aside from what's on the garage door itself, I've been pretty happy with that aspect.I assume the garage doors are insulated? There's a lot of leakage and radiant heat through and around most garage doors. When I ran the calculator to cool my 800 SF garage with 11' ceilings, I ended up with a 24k unit. The calculation came in lower, but when I started answering questions about climate, garage doors, windows, etc., it said I needed a bit over 20k BTU. On an extremely hot day in Phoenix, forget it, I can't get it below 90 degrees out there. On a "regular day" when it's below 105 outside I can cool it just fine. I did the drill and fill for the walls and had 18" sprayed in the attic plus DIY insulated the garage doors.
I don't know man... the system has been checked and double checked and it's working fine, and the building was properly spray foam insulated... nothing I can do at this point but up the cooling system to a higher capacity.My spray foamed building is cooling about 1200 sq feet with 18,000 btu central ac unit. Either the insulation/air sealing is inadequate or that unit isn't working right.
Thanks I considered this, but just not sure how to make it make sense - 24000 downstairs alone is waaaay overkill, and this isn't really any less labor than just running a new line upstairs to the larger air handler. Plus the company is taking back my 24k system so I really can't ask for a better deal than that!Short of replacing EVERYTHING, disconnect the unit upstairs. Install a SECOND compressor and new air handler upstairs.
Blocking the stairwell will help a lot.
EDIT : Move the second air handler downstairs, as far away as possible from the first. Most modern systems have a way to "slave" to units together.
theoldwizard1
Well-known member
Modern mini-splits use inverter drives. It will only consume the power it needs.Thanks I considered this, but just not sure how to make it make sense - 24000 downstairs alone is waaaay overkill,...
American Locomotive
Well-known member
The online calculators are basically useless, and grossly oversize things. 12k BTU should be more than adequate for 600 sq-ft. As others mentioned, most of your cold air is probably just sinking down the stairwell. Another reason why it's probably better to make the "smaller" system work, is smaller capacity minisplits are generally more efficient than larger units.
What might help is keeping your downstairs colder. Try keeping the downstairs at 72 or 74. Keeping it at 78 is just constantly funneling 78+ degree air up stairs. Basically what I'm getting at is, is you might as wall just keep the downstairs colder because you're already making your upstairs system put in a lot of work keeping the downstairs cool. I bet if you sat on the ground at the bottom of the stairs with the system running, you would feel tons of cold air coming down.
What might help is keeping your downstairs colder. Try keeping the downstairs at 72 or 74. Keeping it at 78 is just constantly funneling 78+ degree air up stairs. Basically what I'm getting at is, is you might as wall just keep the downstairs colder because you're already making your upstairs system put in a lot of work keeping the downstairs cool. I bet if you sat on the ground at the bottom of the stairs with the system running, you would feel tons of cold air coming down.
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It's a single garage door, and yes insulated... The downstairs part actually has no problem staying cool - but granted, I keep the temp set higher (78). There's no windows down there aside from what's on the garage door itself, I've been pretty happy with that aspect.
Determining actual running capacity in the field can be difficult. If you really want to know for sure, pick up a 12K BTU window unit on Craigslist and throw it in. See what happens. Will probably cost $100 and you can resell it.
To put your capacity in perspective, my garage is 24 x 24 with a 10' ceiling. I finished the garage and I can tell you my insulation is mediocre. Went out the the garage the other afternoon. Was low 90s outside, mid 80s inside. Turned on my 18K BTU thru wall unit, within hour I was comfortable. It made 72* in around 3 hours. I have no doubt I could get away with 12K, possibly 10K BTUs.
Yeah I mean my garage itself is bigger (25x28 with 9’ ceilings) and does an OK job keeping 78 degrees with a 12000 btu system. But I think you highlighted my point, it’s just OK. It’ll never get down to 72 on a hot day (and we have a lot of really hot days).Determining actual running capacity in the field can be difficult. If you really want to know for sure, pick up a 12K BTU window unit on Craigslist and throw it in. See what happens. Will probably cost $100 and you can resell it.
To put your capacity in perspective, my garage is 24 x 24 with a 10' ceiling. I finished the garage and I can tell you my insulation is mediocre. Went out the the garage the other afternoon. Was low 90s outside, mid 80s inside. Turned on my 18K BTU thru wall unit, within hour I was comfortable. It made 72* in around 3 hours. I have no doubt I could get away with 12K, possibly 10K BTUs.
Not that I need it to…. But I would like the second floor to get lower.
It’s all good guys. I appreciate all the tips, I’ve got a narrow window to take advantage of switching out my current system for the next size up so I’m going to take it, and I will also look into reflective coating or white paint for the roof next summer.
Firebrick43
Well-known member
If it was adequate it wouldn't have those issues. I cool 1450 square feet with 18k btu single unit and it doesn't struggle at all on the hottest days of the year @ or near 100 degrees.Thanks I'll look into those options. We do have a large section of flat / membrane roof, could I basically paint that elastomeric stuff on top of it? It's black now, so I imagine that could help a lot.
Yeah I'm reluctant as well... but also 12000 BTU just doesn't seem like enough. When I talked to the HVAC subcontractor recently he told me that if he had known what we were using the space for he would have recommended 15000 BTU for sure (slams head into wall).
I honestly don't know for sure, but I believe it's adequate, and regardless... no way I could possibly add more.
Yes, system has been professionally inspected and is performing at max capacity.
chrispyny
Well-known member
if your hvac guy is doing the swap out for free, i’d let him go to 30k btu. Then paint the roof with white elastomeric paint like you said. Between the btu’s and white paint, i think you will get the results you want in both the inlaw suite and garage.
if you had to, you could keep the 12kbtu head in the garage and upsize the unit in the inlaw suite. 24/12.
if you had to, you could keep the 12kbtu head in the garage and upsize the unit in the inlaw suite. 24/12.
kj_mustang
Well-known member
I had 3 HVAC contractors come out to give quotes on my build. Only 1 of which did a Manual J calculation and his quote was for a considerably smaller BTU ac unit than the other two. I used him and it has worked flawlessly.
I agree with everyone else saying something else is going on. I've got around 500ish sq ft with a very similar setup and a Gree 9k unit, though I am partially shaded. Insulation is shabby at best and but the 9k unit will put a hurt on that area. It takes about an hour for it to drop from about 85 to 72 on a 90 degree day.
mike93lx
ALLIANCE MEMBER
It's definitely the stairwell. If I open the door to my third floor, the a/c struggles to keep up. Close it and it's oversizedI agree with everyone else saying something else is going on. I've got around 500ish sq ft with a very similar setup and a Gree 9k unit, though I am partially shaded. Insulation is shabby at best and but the 9k unit will put a hurt on that area. It takes about an hour for it to drop from about 85 to 72 on a 90 degree day.
duneslider
Well-known member
So you are keeping one side of the space (lower area garage) at 78 and then trying to keep the upper area at 72 and finding that it isn't working. It sure seems to me the problem is the spaces are NOT isolated. That 78 degree air is moving upstairs all day long. Have you tried keeping the lower temp cooler and seen what happens?
Its like me trying to keep the house cool while my kids leave the door open.
Also, there is no denying you have some stack effect happening. If you aren't willing to do something to stop the stack effect your only option is to throw more HP at it to overcome the issue.
I would be more inclined to add in a second unit just for upstairs since it probably won't need to be run too often, most of the year you won't need to run it.
Its like me trying to keep the house cool while my kids leave the door open.
Also, there is no denying you have some stack effect happening. If you aren't willing to do something to stop the stack effect your only option is to throw more HP at it to overcome the issue.
I would be more inclined to add in a second unit just for upstairs since it probably won't need to be run too often, most of the year you won't need to run it.
chrispyny
Well-known member
You missed the part where he said it’s the best the minisplit could do in the garage.
welder4956
Well-known member
I think that estimate might only be valid for a single level with 8 ft ceilings. You need to break the spaces up based on ceiling height, then add up the total btu required. So for area with 10 ft ceiling multiply by 1.25, for a 12 ft ceiling multiply by 1.5, for a 16 ft ceiling multiply by 2.It's an enclosed space, except that the staircase coming up into the room is an open stairway. I.e. the room has 8' ceilings, but the staircase has a ~16' ceiling or something, the width of the stairs. So that's probably not helping.
Lots of comments about the insulation being inadequate, maybe. I sure don't know, and can't prove that it is or isn't. It was done by a professional spray foam insulation company that supposedly knows what they're doing. All I can tell you is every square inch of that envelope was spray foamed, so there's no point worrying about it because the only way to add more would be to tear out the entire job and start over... and I guess just rebuild the entire building with a different framing structure to allow for more insulation? I don't know - I appreciate the suggestions but there's simply nothing I can do about that now.
I will look into the white paint on the roof, especially if it can prolong the life of the membrane.
Other than that; I think we're just going to have them upsize the system to 15,000 BTU capacity and hope that solves the issue. I know it's a very rough estimate, but from what I've read you multiply the square footage of the room by 25 to get a rough estimate of the capacity needed. 600 sq ft * 25 = 15000 BTU/h. Doing that math backwards, 12000 BTU/h should cover around 480 sq ft. I think taking into account a sun drenched building in a hot climate we're probably operating on a thin margin, so hopefully bumping up in size will get us some more comfort. We'll see I guess, likely won't know til this time next year as it'll be cooling down soon.
kj_mustang
Well-known member
My space is two floors, 9' ceilings on first and vaulted on second floor that go up to about 11' peak. 4' wide stairwell wide open to the high ceiling. 3 windows on second floor facing SW so lots of sun but use curtains. One thermostat on second floor set on 75F right now. Zero issues holding that temp or lower.
Ak Jim
Well-known member
That’s a great idea!How about sealing the door opening with some tape and some plastic to see if it would resolve your cooling issue? If it does you will at least know.
chrispyny
Well-known member
Results?
dcg9381
Well-known member
Agree. I'm in TX. Shop (2400 sqft) is cooled by 2 x 24K units. We're spray foam insulated, but it's "marginal" insulation for the area and we've got massive doors. I'd say when it's 108 outside, we can still cool the shop to mid-70s.I have a hard time believing 12,000 BTU will not cool a 600 sqft area that is properly insulated.
You have 600 feet of foam insulated home on 12k. I think the disadvantage is that you're dealing with the upper floor above a garage. Having heat soaked cars may make the issue worse.
I have a pretty identical setup, 3-car+ garage with a 700 sqft apartment above it. The garage is not air conditioned, but it's cooled by a 1.5 ton HVAC unit and it works fine. Foam insulated.
Did you insulate (with foam) between the garage and the apartment area? Meaning the ceiling of the garage? (Edit, looks like you have open stairs)
Ideals for mitigation:
- Close the stairs off, put in a door.
- If it's related to the cars creating heat, install a small "attic fan" in the garage and pump that heat to the outside. Set it up to turn on automatically with a thermo-switch.
- Can you divert all of your 24k to a new head unit in the apartment? Install a different unit in the garage.
- Add a 2nd mini-split upstairs
75gmck25
Well-known member
Maybe off topic, but - They built you a garage with living space above, but there is no way to isolate the living space from the garage space? This seems like a big fire code violation unless the lower level has also been converted to dedicated living space and you can no longer park a car there. You have a chimney (stairwell) that would carry smoke from a garage fire right up to the living space.